Vale parece ser que el bug viene de la localización y de las apps que las utilizan, de ahí los reboots. El problema de la cámara se me ha solucionado desactivando la localización:
Dear customer, It has come to our notice that users of Mi 11 Ultra may encounter reboot issue in usage scenarios involving GPS after MIUI 12.5 update. If you face this issue, please disable “Location access” in the Settings or Control center as a temporary fix. Sorry for the inconvenience caused. We are now working towards resolving the issue and fix will be rolled out via OTA as soon as possible. Thanks for your understanding and support.